Because I had some problems getting the ATI closed source binary drivers to work on my Gentoo box this info might perhaps help others.
Your mileage may vary of course!

I have an ASUS P5Q Pro motherboard with an Intel P45 chipset if this matters...

1. Log out of your current X seesion, open a console session and power down your X server

Code:

# /etc/init.d/xdm stop

2. Remove the open source driver and select the right OpenGL version

Code:

# emerge -C x11-drivers/xf86-video-ati
# eselect opengl set xorg-x11

3. Set the right VIDEO_CARDS variable in /etc/make.conf and update world to
install x11-drivers/ati-drivers

Code:

# vi /etc/make.conf

VIDEO_CARDS="fglrx"

# emerge -DutNav world

4. Create /etc/X11/xorg.conf

This example is for an ATI Radeon HD 4870 and a 23" Samsung monitor running 32 bit color in full HD resolution with a plain USB mouse and plain PS/2 keyboard, so adjust where needed.
One might also feel the need to add custom font paths.

instead of using a full blown monolithic xorg.conf, you may also use the split one. And from my experience, only the device and dri sections are really required. e.g /etc/X11/xorg.conf.d/10-drivers.conf

I found some little errors in my xorg.conf but they are trivial so it took me a while to notice them...

In the Monitor section VendorName, ModelName and VertRefresh are not options but keywords so they should not be preceded by Option and the keywords don't need double quotes. The vertical refresh rate also does not need double quotes...